It is easy to identify a tree as a member of the genus Quercus
(“oak”), yet often difficult to
distinguish a specimen at the species level. This is especially true for oaks
of the section Lobatae
includes red, black, and Hill’s oak, three species that are prolific at
Pleasant Valley Conservancy. To keep things simple, I will call this the “red
Key taxonomic characteristics of oaks include bud and leaf
morphology, acorn shape (including the cap), and bark thickness and structure. If
the specimen is a “good species”, the characteristics fit well, and a species name
can be attached. But confusion often arises, and this is often due to
It has been known for some years that hybridization is
common in the red oak group, which explains why these species are hard to pin
down taxonomically. It seems reasonable that if two or more of these species
are growing in the same general area, hybridization might occur.
Present day plant taxonomy makes extensive use of DNA
analyses, which provide “ground truth” for traditional morphological taxonomy.
We have been fortunate that botanist Andrew Hipp, Senior
Researcher at the Morton Arboretum, has taken an interest in the oaks at
Pleasant Valley Conservancy. (Andrew worked as a naturalist at the UW-Madison
Arboretum, and received his Ph.D. working on sedges. He wrote an outstanding
book on the Sedges of Wisconsin.)
Since Andrew took up his post at the Morton Arboretum, he
has been working on the taxonomy of oaks, using DNA techniques.
Later he returned with a research team and sampled a
number of oak specimens for DNA analysis.
Owusu, Sandra A., Sullivan, Alexis R., Weber, Jaime A., Hipp,
Andrew L. and Gailing, Oliver. 2015. Taxonomic relationships and gene flow in
four North America Quercus
). “Systematic Botany”
Vol. 40(2): 510-521.
Andrew’s group studied red oak-group specimens from 17
separate geographic sites in the Midwest. My post here deals just with the oaks
he sampled from PVC.
The map here is from Andrew’s paper, with a few labels added
to indicate the approximate locations where the samples were taken. It shows
the distribution of genetically pure, hybrid, and misclassified individuals.
Each specimen is shown with the classification originally made based on
traditional taxonomic criteria. Symbols with open centers indicate that the DNA
data agreed with the taxonomy. If the symbol has a black circle, it means that
the DNA indicated that specimen was a black oak but had been misclassified.
Those with a black star indicate black X red oak hybrids. Those with a white
plus sign are black X Hill’s oak hybrids.
|Figure showing distribution of members of the red oak group from the research paper,|
with labels added to show the approximate location at Pleasant Valley Conservancy.
The table below summarizes the data from this figure.
Analysis of hybridization in the red oak group, based on an analysis of
64 specimens from the north and east side of Toby’s Prairie.
Hill’s oak (Q.
Black misclassified as Hill’s
Black misclassified as Red
In sites such as PVC, where all three species are living
close together, it is perhaps not surprising that hybrids (based on DNA) are common.
A significant number of specimens of Hill’s and black oak had been
misclassified (based on DNA). However, the DNA analysis indicated that quite a
few tree specimens at PVC were not misclassified.
Andrew has now moved on to a study of the bur oak/white oak
group, and will be back this summer to do DNA sampling from some of these